Optically active esters of 2-chloropropionic acid are known chemical intermediates which are used to obtain chemical products useful in the agrochemical industry, particularly in the field of herbicides. Optically active herbicides are advantageous because they are active in dosages which are smaller by one-half than corresponding racemic compounds, thus lessening environmental concerns.
Several processes have already been proposed in order to achieve the synthesis of optically active esters of 2-chloropropionic acid having the highest yields possible.
For example, French Patent FR-B-2,459,221 describes a two-step process wherein, in a first stage, a racemic or optically active alkyl lactic ester is chlorinated in the presence of thionyl chloride and an organic base and then, in a second stage, the reaction mixture resulting from the first stage is heated to a temperature which is at least equal to the decomposition temperature of the alkyl lactate chlorosulfite. In the first stage, a molar excess of thionyl chloride of at least 2.5% relative to the quantity of alkyl lactate introduced into the mixture is maintained, while maintaining a temperature below the decomposition temperature of the alkyl lactate chlorosulfite. European Published Patent Application EP-A-0,163,435 describes the chlorination of alkyl lactate using phosgene in the presence of a tertiary amine. This reaction takes place via a chloroformate intermediate of the formula CH.sub.3 --CH(OCOCl)--CO.sub.2 R, which is then decomposed. However, the disclosed process requires the use and subsequent removal of a large, stoichiometic, quantity of amine. Furthermore, the disclosed process requires a large excess of COCl.sub.2.
German Published Patent Application DE-A-1,135,893 discloses reacting 1-butyn-3-ol with excess phosgene in the presence of approximately 10% of dimethylformamide at a temperature of between 50.degree. and 55.degree. C. to produce the corresponding chlorine compound in a 79% yield. However, because the disclosed process is concerned with racemic compounds, there is no teaching with regard to optical isomerism.
To meet present day requirements, a first objective of the present invention is to provide a process wherein, by starting from an optically active corresponding alpha-carboxylic ester, such as a lactic acid ester, an alpha-chlorocarboxylic ester, such as an ester of 2-chloropropionic acid, may be obtained in an excellent yield, while preserving an excellent optical activity (optical yield).
Another objective of the invention is to provide a process which is simpler than the prior art, especially with respect to the retreatment stage.